2. Field of the Invention
This invention relates to an accessing apparatus which is to be used for an optical information recording and reproducing apparatus doing at least one of recording and reproducing information in an optical recording medium and is to position a light spot on an object track in an optical recording medium.
2. Related Art Statement
Recently, with the development of the information industry, optical information recording and reproducing apparatus such as an optical disk apparatus has come to be noted as a large capacity memorizing (storing) apparatus. In such an optical disc apparatus, as an access system of positioning a light spot on an object track in an optical disk, as shown in the publication, for example, of a Japanese patent application laid open No. 91536/1983, there is a system of counting signals when a track in an optical disk is crossed.
An example of an accessing apparatus of an optical disk is shown in FIG. 9. This apparatus has an optical head 3A recording or reproducing information by forming a light spot by projecting a radiated light on an information track provided on an optical disk as opposed to the optical disk 1 rotated and driven by a spindle motor 2. This optical head 3A is mounted on a positioning means 3 and is moved in the radial direction of the optical disk 1 by this positioning means 3. A tracking error signal showing the position displacement of the light spot on the optical disk 1 is made by a position detecting means 4 connected to the optical head 3A. This error signal is input into a follow-up controlling means 5 and track counting circuit 12. The follow-up controlling means 5 feeds back its output to the positioning means 3 through a controlling loop switching means 11 and forms a follow-up controlling loop so that the optical head 3A may follow any desired track in response to the tracking error signal. In the track counting means 12, the number of tracks to be run will be input from an external apparatus in case the optical head 3A accesses to any desired track. Whenever the optical head 3A crosses the track, the track counting means 12 will reduce the input value. The output of this track counting means 12 is input into a period measuring type speed detecting means 6 and speed instructing means 9. The period measuring type speed detecting means 6 detects the moving speed of the positioning means 3 by measuring the period of crossing the track. The speed instructing means 9 instructs the moving speed of the positioning means 3 in response to the contents of the track counter showing the number of tracks to the momentary object within the track counting means 12. By the error amplifying means 10, the error output of the period measuring type speed detecting means 6 and speed instructing means 9 is amplified, the output is fed back to the positioning means 3 through the controlling loop switching means 11 and a speed controlling loop is formed. The follow-up controlling loop and speed controlling loop are switched with each other by inputting the timing signal into the controlling loop switching means 11 after the contents of the track counter become zero by the track counting means 12.
By the above optical disk accessing apparatus, the speed is controlled according to the optimum speed curve instructed by the speed instructing means 9 to the object track, the loop is switched to the follow-up controlling loop near the object track and the object track is entered to complete the accessing operation.
However, with such a formation as in FIG. 9, as shown in FIG. 10(A), in case noise, as is shown by the broken line, mixes into the zero crossing pulse (track crossing pulse) of the tracking error signal to be the input of the period measuring type speed detecting means 6, the pulse period will be made so short by this noise that, as shown in FIG. 10(B), the output of the speed detecting means 6 will greatly fluctuate to the large speed side as shown by the broken line. As a result, the positioning means 3 will be temporarily greatly decelerated, will move in the reverse direction in the extreme case and will deviate from the speed profile to be inherently followed and the object track will not be able to be reached. Since noise is generated, for example, by the flaw of the disk 1, in order to prevent such a defect, a countermeasure as selecting the disk 1 is necessary. No countermeasure can be taken against the flaw generated during the use.